We usually think of terraforming as something we'll do in the future to other planets, but we have thousands of years of experience changing the shape of our own planet in profound ways.

The term "terraforming" was invented by author Jack Williamson in his 1942 short story "Collision Orbit," published in Astounding Science Fiction. In the intervening decades, its literal meaning ("Earth forming") has shifted. It still commonly refers to the speculative act of altering non-Earth planets to make them habitable by humans. But anything that drastically changes geography to suit human interests can be called terraforming, even if it happens here on Earth. If only we all had the same interests.

Destructive terraforming

Humans have been shaping and changing the Earth for thousands of years, sometimes for the better. All too often, though, our terraforming methods have been destructive – sometimes so destructive that they seem like the opposite of terraforming. Mountain top removal mining, for instance, blows the top off of a mountain and fills a nearby valley with the polluted debris. The resulting blasted landscape looks more like we're turning Earth into Mars than the other way around. Maybe we should call it deterraforming. This series of NASA LANDSAT images shows the Hobet mine gradually obliterating a large swath of West Virginia over the course of about 25 years.

Dams radically alter geography by diverting rivers, creating artificial lakes and changing flood patterns. We've had lots of practice – some Middle Eastern dams are four or five thousand years old, and dams dating to the Roman Empire not only still exist, they still function perfectly well. Modern dams are chart-toppers when it comes to the amount of real estate terraformed. Shasta Dam in California blocks the Sacramento River, creating Shasta Lake. The lake covers almost 50 square miles. What was once a verdant valley ecosystem is now completely under water. Change on that scale has happened at the sites of dozens of large dam projects worldwide.

Dams alter the planet in more subtle ways as well. The power they generate shifts population distribution, and the irrigation that flood control and artificial lakes make possible creates steady, predictable food supplies for growing populations. By enabling population growth, dams boost one of our most overlooked terraforming methods: building cities.

Cities and populations

Cities, of course, aren't built in a few months, and they don't generally change geography instantly. But every city changes the landscape in a thousand small ways that all add up: leveling terrain for construction projects; shifting waterways for drainage; paving over huge areas; tunnel systems for transportation and infrastructure; the heat island effect. If you could somehow strip away the city and see the land beneath, it would look vastly different from how it did before the city was there.

If we talk about cities and population growth as a part of terraforming, we have to talk about the most pervasive, long-term terraforming project ever undertaken – the introduction of huge quantities of greenhouse gases into the atmosphere. That is ultimately how we'll terraform Mars, if we do it, so we've established an interesting test case here on Earth. Increased global temperatures and decreased polar ice levels would be an important first step in terraforming another planet. If we keep at it for another hundred years or so, we'll have a better idea of how it'll play out.

Of course, it's easy to look at all these "detrimental" terraforming methods purely as environmental evils, but everything has a benefit that we're apparently willing to pay the price for. The irrigation, flood control and power generation provided by dams has been significantly helpful for humans. Our desire for cheap on-demand electricity leads to mountain top removal mining, and how many of us would be willing to forego air-conditioning for the next ten years to save a West Virginia mountain?

There's only one sure thing about terraforming: when you change a planet, there will be consequences, and not always the consequences you expect. We'll get deeper into unintended consequences in part two of this series, when we examine more constructive terraforming methods.